Conventional power supply controllers use metal-oxide semiconductor field-effect transistor (MOSFET) transistors as switches such as, for example, U.S. Pat. No. 6,972,969, U.S. patent application Ser. No. 11/745,299 filed on May 7, 2007 and U.S. patent application Ser. No. 11/558,326 filed on Nov. 9, 2006 which are incorporated by reference herein in their entirety.
However, the output current limit control techniques used in conventional systems can not be directly used when a bipolar junction transistor (BJT) switching device is used instead of a MOSFET switching device, and when the pulse width modulation (PWM) switching frequency is relatively high, such as 65 kilohertz (kHz) and above. If the PWM control is used for a wide range of the constant current (CC) mode, which has fixed PWM switching period, in order to maintain the constant output current, then when the output voltage is reduced (from 5 volts (V) towards 1V for example), the switch's ON time (Ton) is reduced accordingly. As a consequence, when the output voltage is low, the Ton time can become very small. For power MOSFET's which typically have short turn-on and turn-off times, this is typically not a problem. However, when a BJT is used, the very-narrow Ton time can cause control problems and degrade the control accuracy of the CC current limit, because the BJT's normally have much longer turn-on and turn-off times which can be dynamically changed with line voltage and load. Furthermore, because of the longer turn-on time, this effectively becomes a minimum Ton time. This minimum Ton time requirement also affects the digital control resolution requirements.
Another conventional CC control technique is accomplished by using constant current (CC) Pulse-Frequency-Modulation (PFM) control. By using the PFM for the CC control, the Ton time is substantially fixed, and the switching period is gradually increased when the output voltage is reduced. In this CC PFM control technique, the output current can still be controlled constantly, without causing the problem of having a very-narrow (short) Ton time. A problem with conventional constant current PFM control, however, is that when there is still significant energy being transferred from the transformer primary side to the secondary side, the switching frequency can be reduced to a level that results in sounds that can be heard by humans, such as below approximately 30 kHz with a constant current voltage of approximately 1 ampere (A) when the output voltage is above 1V. This kind of acoustic noise is not acceptable in some applications such as chargers, and it requires special process during transformer manufacturing to eliminate the noise which increases the cost and complexity.